Following the spread of high-vision broadcasting and the development of techniques for recording/reproducing data, such as dynamic and static images, as digital information on/from a recording medium, BD (Blu-ray disc), which can record or reproduce larger volumes of information than the typical optical disks such as DVD and CD, have recently come into wide use.
In order to refine further the high-vision images and realize recording and reproduction of three-dimensional stereo images, optical information recording and reproduction systems are needed that can record and reproduce information in volumes even larger than those of BD.
In a typical optical recording/reproduction system that is presently used for optically recording and reproducing information signals on/from a recording medium, laser light for reproduction is focused on an optical disk, which is an optical recording medium and the reflected light therefrom is monitored to read the information signals that have been recorded on the optical disk.
Further, when information signals are recorded on an optical disk, or when the information signals recorded on the optical disk are deleted, the laser light for recording or laser light for deleting with a predetermined laser power is focused on a desirable position on the optical disk, the temperature of the region that is the object of recording or deletion on the information recording layer of the optical disk is raised due to irradiation with the laser light for recording or laser light for deletion, and the information signal is recorded or deleted.
In the conventional optical recording/reproduction technique, the density of data recorded on the optical disk depends on the spot diameter of laser light focused on the recording surface of the optical disk and data of higher density can be recorded or reproduced by decreasing the spot diameter of the laser light.
The minimum spot diameter of laser light focused by the objective lens is proportional to the laser light wavelength and inversely proportional to the numerical aperture of the objective lens. For this reason, the laser source wavelength is decreased and the numerical aperture of the objective lens is increased in order to increase the recording density by reducing the spot diameter of laser light.
Where the numerical aperture of the objective lens is denoted by NA, the angle of incidence of the focused light is denoted by θ, and the refractive index of the medium where the light is focused is denoted by n, the numerical aperture can be represented as NA=n×sin θ. This equation indicates that it is essentially impossible to increase the numerical aperture NA above 1 when the focusing path is provided in the air with a refractive index n of 1.
An optical pickup of a near-field optical recording/reproduction system that uses a solid immersion lens has recently been suggested as a technique for overcoming the aforementioned numerical aperture limitation.
A solid immersion lens has a spherical surface portion and a flat surface portion, the flat surface portion is formed in a shape constituting part of the sphere, and the flat surface portion is disposed in very close proximity to the surface of the optical recording medium. At the boundary surface of the solid immersion lens and the optical disk, an evanescent wave is transmitted and this evanescent wave reaches the information recording layer of the optical disk.
When the numerical aperture is thus made more than 1, where the thickness of air layer is not controlled to a sufficiently small level, the recording accuracy or reproduction accuracy is degraded by the decrease in the intensity of laser light focused on the information recording layer of the optical disk. Thus, for example, the recording accuracy or reproduction accuracy is degraded as mentioned hereinabove unless the gap between the solid immersion lens and the optical disk is made equal to or less than 100 nm, desirably equal to or less than about 50 nm.
Accordingly, Patent Literature 1 describes a method for controlling the gap between the solid immersion lens and the optical disk with such high accuracy.
The optical pickup disclosed in Patent Literature 1 will be described below with reference to FIG. 13. FIG. 13 illustrates the configuration of the conventional pickup.
The conventional optical pickup is configured by providing a solid immersion lens 101 having a spherical surface portion and a flat surface portion parallel to the surface of the optical information medium 108, as shown in FIG. 13.
The solid immersion lens 101 is for example of a semispherical shape and has a thickness substantially equal to the radius of the sphere. The distance (gap) between the flat surface portion of the solid immersion lens 101 and the surface of the optical information medium 108 is maintained by a servo-mechanism at about 1/10 of the emission wavelength of a semiconductor laser 103 serving as a light source.
In the conventional optical pickup, the gap error signal corresponding to the distance between the surface of the optical information medium 108 and the flat surface portion of the solid immersion lens 101 is obtained by detecting a component with a polarization state orthogonal to the polarization state of the reflected light at the time the distance between the surface of the optical information medium 108 and the flat surface portion of the solid immersion lens 101 is zero, in the reflected light (returned light) that has been obtained by emission from the semiconductor laser 103 and reflection by the optical information medium 108.
Thus, in the conventional optical pickup, the light flux emitted from the semiconductor laser 103 is converted by a collimator lens 104 into a parallel light flux that falls on a beam splitter 105.
The light flux emitted from the semiconductor laser 103 is transmitted by the beam splitter 105 and then falls on a polarization beam splitter 106. The light flux emitted from the semiconductor laser 103 becomes P polarized light with respect to the reflective surface of the polarization beam splitter 106. The P-polarized light flux is transmitted by the reflective surface and transmitted by the polarization beam splitter 106.
The light flux transmitted by the polarization beam splitter 106 is transmitted by a quarter-wavelength plate 107, which is disposed so that the crystal axis thereof is inclined at an angle of 45° with respect to the incident polarized light direction, and converted from linearly polarized light into circularly polarized light that falls on an objective lens 102. The objective lens 102 converges the incident parallel light flux and the converged light flux falls on the solid immersion lens 101. In the solid immersion lens 101, a focal point is formed close to the flat surface portion located in close proximity and parallel to the surface of the optical information medium 108. The refractive index of the solid immersion lens 101 is for example 1.8.
The focused light flux is focused as an evanescent wave on the information recording layer of the optical information medium 108. In this case, the numerical aperture of the objective lens 102 is for example about 1.36.
The optical pickup shown in FIG. 13 reproduces information signals from the optical information medium on which the information signals have been recorded by recording pits (concave and convex marks) or the optical information medium on which information signals have been recorded by using phase changes. Thus, the light flux focused on the information recording layer of the optical information medium 108 undergoes reflection that differs depending on the presence or absence of the recording pits on the information recording layer and returns to the polarization beam splitter 106 via the objective lens 102 and the quarter-wavelength plate 107.
The light flux that has been reflected by the optical information medium 108 and returned to the objective lens 102 is converted from the circularly polarized light into linearly polarized light by transmission through the quarter-wavelength plate 107. The polarization direction in this case is orthogonal to the polarization direction of the light flux emitted from the semiconductor laser 103. Therefore, the light flux that has been reflected by the optical information medium 108 and transmitted by the quarter-wavelength plate 107 becomes S polarized light with respect to the reflective surface of the polarization beam splitter 106. The S polarized reflected light is reflected by the reflective surface of the polarization beam splitter 106, comes off the optical path that returns to the semiconductor laser 103, and is received by the first detector 109 for obtaining the reproduction signal from the optical information medium 108.
In the conventional optical pickup, in a plane 111 perpendicular to the optical axis between the beam splitter 105 and the polarization beam splitter 106, the light flux emitted from the semiconductor laser 103 is linearly polarized light having only a field component in the same direction as that of the semiconductor laser 103.
In a state in which the flat surface portion of the solid immersion lens 101 is brought into intimate contact with the surface of the optical information medium 108, practically the entire reflected light becomes light in which the polarization direction has been rotated through 90° by back-and-forth movement though the quarter-wavelength plate 107. Therefore, a light flux with a distribution substantially identical to that of the light emitted from the semiconductor laser 103 falls on a plane 112 immediately in front of the first detector 109, that is, the plane 112 perpendicular to the optical axis between the polarization beam splitter 106 and the first detector 109. In this case, the reflected light from the optical information medium 108 practically does not return to the plane 111 between the beam splitter 105 and the polarization beam splitter 106.
Further, in a state in which the flat surface portion of the solid immersion lens 101 is separated from the surface of the optical information medium 108, the light incident at an angle greater than the critical angle of the flat surface portion, that is, the light with a numerical aperture greater than 1 (n×sin θ>1, n is a refractive index of the solid immersion lens; θ is an incidence angle of the light that will be focused), of the light that is focused in the vicinity of the flat surface portion of the solid immersion lens 101, is reflected by the flat surface portion.
Thus, the polarization state of the light reflected by the flat surface portion of the solid immersion lens 101 rotates due to the total reflection. Further, the light that underwent total reflection by the flat surface portion of the solid immersion lens 101 includes a polarization component perpendicular to the polarization direction of the reflected light obtained when the flat surface portion of the solid immersion lens 101 is in intimate contact with the surface of the optical information medium 108 as described hereinabove. Therefore, when the flat surface portion of the solid immersion lens 101 is separated from the surface of the optical information medium 108, the distribution of the returned light on the plate 111 between the beam splitter 105 and the polarization beam splitter 106 corresponds to a state in which only the peripheral portion of the light flux has returned.
The light that has thus returned to the plane 111 is reflected by the reflection surface of the beam splitter 105 and received by the second detector 110 for obtaining a gap error signal, as shown in FIG. 13. The gap error signal is a signal corresponding to the distance between the flat surface portion of the solid immersion lens 101 and the surface of the optical information medium 108.
In this case, the distribution of the returned light in the plane 112 immediately in front of the first detector 109 corresponds to a state in which the peripheral portion of the light flux is lost.
In the relationship between the light quantity received by the second detector 110 and the distance (air gap) between the flat surface portion of the solid immersion lens 101 and the surface of the optical information medium 108, where the position of the solid immersion lens 101 in the direction of approaching the optical information medium 108 and withdrawing therefrom is controlled so as to maintain the ratio of the light quantity at the second detector 110 to the incident light quantity at 0.2, the distance (air gap) between the flat surface portion of the solid immersion lens 101 and the surface of the optical information medium 108 can be maintained at 1/10 the wavelength.
Optical disks with a multilayer structure having two or three or more information recording layers have recently been suggested for use in optical information recording/reproduction systems using solid immersion lens, such as shown in FIG. 13, in order to increase further the capacity.
The problem encountered when information is recorded on or reproduced from such a multilayer optical disk in the optical pickup with a structure shown in FIG. 13 is that the reflected light from the information recording layers other than the object information recording layer used for recording or reproducing information is superimposed on the reflected light from the object information recording layer, and these reflected lights interfere with each other, thereby causing errors in the reproduction signal and gap error signal.
Further, the distance between the information recording layer used for recording or reproducing information and the flat surface portion of the solid immersion lens 101 increases. Therefore, the problem encountered in the case of the structure shown in FIG. 13 is that the focal point position of the reflected light from the light spot focused on the information recording layer and the focal point position of the reflected light from the flat surface portion of the solid immersion lens 101 shift significantly from each other, thereby narrowing the operation range of the gap error signal and making it impossible to detect correctly the gap position.
CITATION LIST
Patent Literature
Patent Literature 1: Japanese Patent Publication No. 4228666